What we’ve been told, and this is from a small number of operators but it sounds reasonable to me, is that if the principal owner is onboard then an attendant isn’t necessary (though they may want one anyway, if they had guests, for example) unless the flight is transatlantic (if I remember correctly). However, if it’s charter passengers then an attendant will usually be there to help them with any questions, service, and well, to make sure they behave.